Light modulating medium for image projection apparatus

ABSTRACT

ADDITION OF SMALL AMOUNTS OF TRIS(3-PHENYLPROPYL) PHOSPHATE TO A LIGHT-MODULATING FLUID INCREASES THE CRITICAL QUIETING THICKNESS OF THE FLUID THEREBY PERMITTING A BRIGHTER IMAGE TO BE PROJECTED BY THE PROJECTION APPARATUS IN WHICH THE MODIFIED FLUID IS USED AS THE LIGHT-MODULATING MEDIUM.

United States Patent O 3,766,081 LIGHT-MODULATING MEDIUM FOR IMAGE PROJECTION APPARATUS David A. Orser, Liverpool, N.Y., assignor to General Electric Company No Drawing. Filed May 18, 1972, Ser. No. 254,696 Int. Cl. F21v 9/00; H04n 3/ 16, 5/44 US. Cl. 252-300 2 Claims ABSTRACT OF THE DISCLOSURE Addition of small amounts of tris(3-phenylpropyl) phosphate to a light-modulating fluid increases the critical quieting thickness of the fluid thereby permitting a brighter image to be projected by the projection apparatus in which the modified fluid is used as the light-modulating medium.

This invention relates to an improved fluid, light-modulating medium for the projection of self-erasing, rapid decay images in apparatus of the kind wherein a fluid, light-modulating medium is deformed into defraction and/ or refraction gratings by the impression of electron charges thereon as a function of electrical signals corresponding to the images. By the use of these improved fluids, improved electronic projection systems are provided which are capable of projecting a brighter image.

Apparatus employing such a fluid, light-modulating medium is described, for example, in US. Pat. 2,943,- 147-Glenn, Jr., wherein is disclosed a projection system comprising an evacuated glass envelope containing an electron gun for producing an electron beam, which is deflected in a rectangular raster over the surface of a light-transmitting, electron-deformable, light-modulating medium contained within a portion of the transparent glass envelope. The electron beam is modulated, as by a television signal applied to the deflection means. The deflected electrons strike a portion of the light-modulating medium in the raster area traversed by the electron beam, and these electrons are electrically attracted to a conductive coating over which the light-modulating medium is supported. As these electrons strike the surface of the medium, they produce deformations in this surface with the amplitude of the deformations being a function, along with other parameters, of the number of electrons deposited by the electron beam at various points over the surface of the raster area. As a result, the amplitude of these deformations are a function of the electron beam modulation. Repetitive rates of more than one image per second are employed and this is possible because of the rapid decay of each image such that the same area of fluid is employed for the formation of a quick sucession of images.

Conventional deformable light-modulating mediums are described in the aforesaid Glenn patent and improved mediums are described in US. Pats. 3,288,927Plump, 3,317,664 and 3,317,665Perlowski, Jr., and 3,541,992 Herrick et a1. All of these patents are hereby incorporated by reference for a discussion of the properties required in the light-modulating fluid and .for a showing of the type of apparatus and a description as to how it operates in projecting an image utilizing either the prior art, fluid, light-modulating medium or the fluid, light-modulating medium of the instant invention. The actual operation of the apparatus and the way in which these fluids are employed therein does not form a part of the instant invention, except to the extent that the improved properties of my fluid result in an improved projection apparatus in which they are used, which is more fully disclosed and discussed hereinafter.

The fluid, light-modulating medium is a thin layer of light-transmittable fluid in which the electron beam forms phase diffraction gratings and/or refraction gratings in the form of alternate hills and valleys caused by the deforming effect of the electron beam. The adjacent valleys are spaced apart by a predetermined distance such that each portion of light incident on a respective small area or point of the medium is deviated in a direction orthogonal to the direction of the valleys. The intensity of the deviated light is a function of the depth of the valley and diminishes as autoerasure of the deformation occurs wherein the hills and valleys just diminish and the fluid is ready for the writing of a new image.

It has been found that relatively thick layers of lightmodulating fluid are necessary to achieve uniformly good deformation or writing characteristics. Unfortunately, however, it has been shown that when electron charges are deposited on thick layers of fluid to produce desired deformation therein, additional unwanted deformation hearing no relationship to the desired deformations are created. These latter deformations, hereinafter referred to as noise, are appreciable in depth in relation to the desired deformations and are substantial in extent. As a result, they produce deviation of light which deleteriously affects the contrast in the projected image and these deviations become a part of the projected image destroying the distinctness of image boundaries. These unwanted deformations can vary in their effect from causing a haziness to actually obliterating all traces of the signal modulation. For a particular light-modulating fluid, if the thickness of the fluid in the raster area is reduced below a certain critical value, these unwanted deformations do not occur. This critical value is referred to as the critical quieting thickness, hereinafter for brevity sake designated as CQT. Also, it has been found that CQT values vary in inverse relationship to the current of the electron beam, i.e., for smaller electron beam currents, the CQT has a larger value.

Such noise has not been a problem in thermoplastic recording such as described in US. Pat. 3,113,179- Glenn, Ir., because after its deformation the deformed medium carried by the recording tape is moved away from the electron beam and is changed to the solid state before noise has had time to form therein. As described hereinabove, the fluids to which the instant invention apply provide rapidly decaying images, which automatically erase themselves or are erased by the combined effects of the surface tension of the fluid medium and an imposed force, and the same portion of fluid receives a new image as the fluid remains in the fluid state. In the latter case, the writing is of a rapidly repetitive nature while in the former case the writing is nonrepetitive.

The depth of fluid at which the CQT occurs becomes manifest in the following manner. When the thickness of the fluid is less than the CQT, each fluid has its own capacity to conduct electric charges impressed on its surface through the fluid to the ground plane base, i.e., the conductive layer. It is believed that this charge transfer occurs both by electrical conduction phenomena and by various flow patterns coordinated with the raster lines. If the thickness of the fluid is greater than the CQT, flow patterns uncoordinated with the raster lines or with the modulated signals are generated in the fluid. These uncoordinated flow patterns deform the surface resulting in the deviation of light not resulting from signal modulation. This phenomenon is known as optical noise or noise. For a given value of thickness of the fluid layer, this uncoordinated flow occurs as the current density of the electron beam is increased from zero to some value beyond which a sudden and widespread initiation of uncoordinated flow occurs in the fluid. The fluid thickness at this depth is at the CQT for this value of current.

Operation with a fluid thickness of less than the CQT value is desirable because the noise referred to hereinabove is destructive of the signal-modulated image. More effective operation, greater flexibility for this system and an improved image production are achieved by increasing this value of the CQT whereby operation with a thickener layer of fluid without noise is made possible.

Herrick et al. in their U.S. Pat. 3,541,992, referenced above, obtained a light-modulating fluid of improved CQT properties by dissolving various polymers in the conventional fluids of the prior art to obtain a fluid having viscoelastic properties. The preferred combination was a polybenzyltoluene fluid in which polystyrene was dissolved. The amount of polystyrene that had to be dissolved to give the desired viscoelastic behavior was dependent upon the linearity of the polystyrene as well as upon its molecular weight. Because of this variation, they disclose a very simple test for determining whether or not a particular fluid had viscoelasticity by dipping a pointed probe into the liquid and then withdrawing it. If the liquid does not have their requisite viscoelasticity, no connection remains between the end of the probe and the surface of the liquid upon withdrawal of the probe. If the liquid does have the desired viscoelasticity, a thread or filament having a length-to-diameter ratio of about 100 to 1, or greater remains, connecting the probe and the surface of the liquid as the probe is moved away from the surface. Commonly such threads could be drawn to lengths of over /2 inch. As a practical consideration, the thread test is conducted at the operating temperature at which the fluid is to be used in the projection system.

Although such fluids do have an improved CQT, it would be highly desirable to still further increase this property of the light-modulating fluids so that a still further increase in the fluid thickness can be used in the projection system without initiating noise, thereby resulting in an improved projection system.

It is therefore a prime object of this invention to provide a light-modulating fluid of considerably improved CQT properties whereby optical noise therein caused by uncoordinated flow patterns is eliminated.

It is also an object of this invention to obtain the additional advantages to be gained from increased CQT in light-modulating fluids whereby the optical etficiency of the projection system will be increased and the rise time of the fluid will be decreased to bring it into better operational balance with the decay time.

It is another object of this invention to provide an improved light-modulating fluid yielding the capacity for greater light efliciency for the projection system with minimal additional damage to the fluid writing medium.

These objects and other advantages are obtained by dissolving tris(3-phenylpropyl) phosphate in the lightmodulating fluid used in the projection of self-erasing rapid decay images. Although my invention is applicable to any of the particular light-modulating fluids described in the above Herrick Pat. 3,541,992, the one fluid which possesses the best combination of desirable properties for a light-modulating fluid is polybenzyltoluene containing dissolved polystyrene. The polybenzoyltoluene fluid can be the polybenzyltoluene fluid obtained by condensing benzyl chloride with toluene in the presence of a Friedel- Crafts catalyst to produce a fluid having a viscosity of at least 100 centistokes (cs.) at 25 C. which is separated from the lower boiling fractions by distillation. Such a fluid is described in the above-referenced Plump Pat. 3,288,927. A preferred and improved polybenzyltoluene is prepared by condensation of benzyl alcohol with toluene in the presence of an acidic catalyst as described in the copending application of Charles E. Timberlake, Ser. No. 92,178, filed Nov. 23, 1970 and assigned to the same assignee as the present invention. This material, likewise, is distilled to provide a fraction having a viscosity of at least 100 centistokes at 25 C.

Thes fluids are modified by dissolving an amount of polystyrene sufficient to attain the desired viscoelastic behavior in the fluid. Simultaneously, or as a separate step,

tris(3-phenylpropyl)phosphat is dissolved in the fluid. The amount of the latter material is determined by the properties one desires to obtain. Very small amounts in the order of 0.1-1 percent will have some effect on increasing the CQT of the fluid but still further gains are obtained by use of still larger amounts. However, the further gain in CQT must be measured against the fact that larger amounts of the tris(3-phenylpropyl)phosphate decrease the viscoelastic nature of the fluid. This can be compensated to some extent by increasing the amount of polystyrene added or the molecular weight of the polystyrene used can be increased. To obtain a balance between the requisite viscoelectric behavior and the desired increase in CQT, amounts up to about 6 percent can be used but the best all-around balance of desirable properties are usually obtained at a concentration of about 4 percent by weight. However, these percentages are not fixed and vary dependent upon the viscosity of the particular polybenzyltoluene used, both the molecular weight and degree of branching of the polystyrene used and the amounts of polystyrene dissolved in the polybenzyltoluene fluid. Therefore, the amount of polystyrene dissolved in the polybenzyltoluene is best described as that amount suflicient to provide viscoelastic behavior to the fluid and the amount of tris(3-phenylpropyl)phosphate is best described as the amount sufficient to increase the critical quieting thickness of the fluid but insufficient to destroy the viscoelastic behavior of the fluid. There is no need to use larger amounts of either of these materials than is required to obtain the desired properties.

It was indeed surprising to find that other phosphate esters had drawbacks compared to tris(3-phenylpropyl) phosphate. It was found that the normal triarylphosphates were not as radiation resistant to the electron beam. The trialkylphosphates decreased the ability of the light-modulating fluid to wet the conductive layer. The lower aralkyl phosphates such as tribenzylphosphate had the same disadvantages as the triarylphosphates. However, it was found that they were all capable of increasing the CQT of the polystyrene modified polybenzyltoluene fluid. These compounds therefore could be used to increase the CQT providing the disadvantages of their use could be tolerated. However, where the best fluid, lightmodulating medium is desired, tris(3-phenylpropyl)phosphate should be used.

In order that those skilled in the art may better understand my invention, the following examples are given by way of illustration and not by way of limitation.

EXAMPLE 1 A polybenzyltoluene fluid in which there was dissolved 2 percent by weight polystyrene was used as the base fluid, light-modulating medium. Using the procedure given in U.S. Pat. 3,541,992, the CQT of the base fluid Was found to be 12.711. at 1000 centistokes. When 4% tris(3-phenylpropyl)phosphate was dissolved in this same base fluid, the CQT was increased to 16p. at 1000 centistokes, an increase of 26%.

EXAMPLE 2 When Example 1 was repeated, except using a different polybenzyltoluene in which 2% polystyrene was dissolved, the critical thickness increased from 13.1 1 at centistokes to 17.3, at 1000 centistokes, an improvement of 32%, by dissolving 4% by weight tris(3-phenylpropyl) phosphate in the polystyrene modified fluid.

The increase in the CQT obtained by this invention permits these fluid, light-modulating mediums to be used in a thicker layer in the electronic projection systems thereby resulting in an improved system because of the increased brightness of the image which can be projected therefrom.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A composition of matter consisting essentially of a polybenzyltoluene fluid having a viscosity of at least 100 centistokes at 25 C. in Which there is dissolved a suflicient amount of polystyrene to provide viscoelastic behavior to said fluid and a sufficient amount of tris(3- phenylpropyl)phosphate to increase the critical quieting thickness of said fluid but insuflicient to destroy the viscoelastic behavior of said fluid.

2. In a fluid, light-modulating medium for use in apparatus in which a thin layer of said fluid, light-modulating medium is supported on a conducting plane located relative to means for producing an electron beam so that said beam is directed at said plane to build up charge in said fluid, light-modulating medium, which charge produces self-erasing deformation in the surface of said layer, each said deformation acting to dilfract light directed at said layer from a light source in a light optical system, the

diffracted light being projected by the optical system as a 15 function of each deformation to form self-erasing, rapid, decay images, the improvement wherein said fluid is the composition of claim 1.

References Cited UNITED STATES PATENTS 3,541,992 11/1970 Herrick etal. 178-75 D NORMAN G. TORCHIN, Primary Examiner J. P. BRAMMER, Assistant Examiner US. Cl. X.R. 178-7.5 D 

